It is also known to decompose the exposure into several passes. The additional exposure step is then applied in general to finer geometries so as notably to improve the method resolution and window. One then speaks of eRIF (electron Resolution Improvement Feature) function or method step. Methods of this type have been described notably by Martin and Manakli (“New writing strategy in electron beam direct write lithography to improve critical dense lines patterning for sub-45 nm nodes”—L. Martin—EMLC 2009; international patent application No. PCT/EP2011/055746 with the title “Procédé de lithographie électronique avec correction des arrondissements de coins” [Method of electron lithography with correction of the roundings of corners], of which S. Manakli is the inventor and whose proprietor is one of the co-applicants of the present patent application.
The separate computation of the parameters of these various methods leads however to a solution which is sub-optimal both as regards design time and as regards exposure time. Solutions with joint computation of the dose modulation and geometric correction parameters have already been proposed. One avenue is that proposed by international patent application No. PCT/EP2011/055863 with the title “Procédé de lithographie à optimisation combinée de l'énergie rayonnée et de la géométrie de dessin” [Method of lithography with combined optimization of the radiated energy and of the design geometry], of which S. Manakli is the inventor and whose proprietor is one of the co-applicants of the present patent application. According to this method, the energy radiated in a dose applied to a zone and the dimensions of the pattern to be etched on this zone are computed in combination by way of the energy latitude of the method. This method works well for simple patterns such as cells with large mesh size, for the ends of lines and corners, notably. It is less effective for more complex patterns. Another avenue for jointly optimizing the radiated dose and projected geometry parameters consists in minimizing the discrepancy between the result of convolving a radiated dose with the PSF and the target pattern. Accordingly, the suggestion has been made of deconvolving the pattern to be etched by an appropriate procedure. The use of inverse Fourier transforms in combination with a two-Gaussian conventional PSF such as proposed by Eisennmann (reference cited hereinabove) has been advocated by Haslam (“Transform based proximity corrections: Experimental results and comparisons”—M. E. Haslam, J. F. McDonald, Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, N.Y.—J. Vac. Sci. Technol. B4(1), January/February 1986). However, at the dimensions relevant for the technologies currently in production or under development (critical dimension of one to two tens of nanometers), this procedure is no longer suitable because of the cutoff, inherent to the inversion, that it carries out of the high spatial frequencies, which prevents sufficiently precise account being taken of the forward scattering effects which dominate at these distances.